Zilog Z80380 - Manual (Page 31)

4-6
Z380 ™
U SER ' S M ANUAL
DC-8297-03
Z ILOG
4.2.6 Program Counter Relative Mode (RA) (Continued)
PCz PC
Before instruction execution 0000 1000
After instruction execution 0000 0FFE
Address calculation: In Native mode, –2 is encoded as
0FEH in the instruction, and it is sign extended to a 16-bit
value before added to the Program Counter. Calculation is
done in modulo 2
16 and does not affect the Extended
portion of the Program Counter.
1000
+ FFFE
FFFE
Address calculation: Since this is a 4-byte instruction, the
PC value after fetch but before jump taking place is:
19590807
+ 00000004
1959080B
The displacement portion, –5000H, is sign extended to a
32-bit value before being added to the Program Counter.
Calculation is done in modulo 2
32 and affects the Extended
portion of the Program Counter.
1959080B
+ FFFFB000
1958B80B
2. Jump relative in Extended mode, 16-bit displacement
SETC XM ;Put it in Extended mode of operation
JR $-5000H ;Jumps to the location
;(Current PC value) – 5000H
;$ stands for current PC value
;This instruction jumps to itself.
PCz PC
Before instruction execution 1959 0807
After instruction execution 1958 B80B